I don't know...you probably came across my thread at some point, I had pressure remaining overnight but all tests to my head were fine and I've gone over 1500 miles since then without a problem. All posts immediately suggested a bad head gasket but I was doubtful, I don't think my car woulld still be running if I had a bad head gasket that long ago. Oil in the coolant is evidence of much more of a problem however...
The bottom part of your post sounds like normal operation to me, system builds pressure and then loses it, not sure what is up with the warning light
, maybe an air bubble in the system.